Step stool



FIG. 1 is a right side elevational showing my new design the opposite side being a mirror image;

FIG. 2 is a bottom plan view;

FIG. 3 is a front elevational view;

FIG. 4 is a rear elevational view;

FIG. 5 is a top plan view; and

FIG. 6 is a top rear perspective view thereof. 

The ornamental design for a step stool, as shown and described. 